The present invention relates to displacement control valves employed in variable displacement compressors to control compressor displacement. More particularly, the present invention relates to control valves for controlling the amount of refrigerant gas flowing into a control chamber from a discharge pressure zone in a variable displacment compressor.
A variable displacement compressor generally has a housing that housed a control chamber and a rotatable drive shaft. Cylinder bores extend through a cylinder block, which forms part of the housing. A piston is retained in each cylinder bore. A swash plate is supported on the drive shaft in the control chamber. The swash plate is permitted to incline with respect to the drive shaft while rotating integrally with the drive shaft. Each piston is coupled to the swash plate. The rotation of the swash plate reciprocates each piston. This draws refrigerant gas into the associated cylinder bore from a suction chamber, compresses the refrigerant gas, and then discharges the compressed refrigerant gas into a discharge chamber. The inclination of the swash plate is altered in accordance with the difference between the pressure of the cylinder bores and the pressure of the control chamber. The inclination of the swash plate is smaller when the pressure difference is larger. That is, the inclination of the swash plate decreases as the pressure of the cylinder bores becomes smaller relative to the pressure of the control chamber. A decrease in the inclination of the swash plate shortens the stroke of the pistons and decreases the displacement of the compressor.
A typical variable displacement compressor incorporates a control valve to control the amount of refrigerant gas flowing between the control chamber and the discharge chamber. For example, a control valve 1, which is shown in FIG. 6, is employed to control the amount of refrigerant gas that flows into the control chamber from the discharge chamber. A pressure sensing element, or diaphragm 2, is housed in the control valve 1. An atmospheric pressure chamber 8 is defined on one side of the diaphragm 2 and a suction pressure chamber 9 is defined on the other side of the diaphragm 2. The pressure of the suction chamber (suction pressure) is communicated to the suction pressure chamber 9. A spring 3 is arranged in the atmospheric pressure chamber 8. The control valve 1 further includes a valve hole 5 and a solenoid 6. The opened area of the valve hole 5 is adjusted by a valve body 4, which is connected to the diaphragm 2. The diaphragm 2 urges the valve body 4 in a direction opening the valve hole 5. A further spring 10 urges the valve body 4 in a direction closing the valve hole 5. The solenoid 6, when excited, also urges the valve body 4 by means of a rod 7 in the direction closing the valve hole 5 with a force corresponding to the current fed to the solenoid 6.
The opened area of the valve hole 5 is determined by the position of the valve body 4 with respect to the valve hole 5. The position of the valve body 4 is determined by the balance between the force that urges the valve body 4 away from the valve hole 5, or in an opening direction, and the force that urges the valve body 4 toward the valve hole 5, or in a closing direction. The force urging the valve body 4 in the opening direction is produced by the difference between the force applied to one side of the diaphragm 2 by the pressure in the atmospheric pressure chamber 8 and the force of the spring 3 and the force applied to the other side of the diaphragm 2 by the suction pressure in the suction pressure chamber 9. The force urging the valve body 4 in the closing direction is produced by the sum of the force of the solenoid 6 and the force of the spring 10.
When the current fed to the solenoid 6 is maintained at a constant value, that is, when the force of the solenoid 6 is constant, the valve body 4 moves in accordance with the fluctuation of the suction pressure. More specifically, an increase in the suction pressure decreases the opened area of the valve hole 5 and decreases the amount of refrigerant gas sent to the control chamber from the discharge chamber. This lowers the pressure of the control chamber, decreases the difference between the pressure in the control chamber and the pressure in the cylinder bores, and increases the inclination of the swash plate. As a result, the displacement of the compressor increases, which gradually decreases the suction pressure. On the other hand, a decrease in the suction pressure increases the opened area of the valve hole 5. This decreases the displacement of the compressor, which gradually raises the suction pressure. Accordingly, the control valve 1 functions to maintain the suction pressure in a constant state.
The control valve 1 also adjusts the target value of the suction pressure in accordance with the value of the current fed to the solenoid 6. For example, if the force of the solenoid 6 increases as the value of the current flowing through the solenoid 6 increases, the force urging the valve body 4 in the closing direction increases. Accordingly, the opened area of the valve hole 5 decreases and lowers the pressure of the control chamber. This gradually decreases the suction pressure. In other words, the control valve 1 maintains the suction pressure at a lower value, or lowers the target value, as the value of the current fed to the solenoid 6 increases. Accordingly, the control valve 1 functions to alter the target suction pressure in accordance with the value of the current fed to the solenoid 6.
The force of the solenoid 6 must be permitted to change within a range that corresponds to the fluctuation range of the suction pressure in the suction pressure chamber 9. In other words, the value range of the current fed to the solenoid 6 must be substantially proportional to the fluctuation range of the suction pressure. Japanese Unexamined Patent Publication No. 8-110104 describes a compressor that employs carbon dioxide (CO.sub.2) as a refrigerant. In such a compressor, the pressure of the refrigerant is ten or more times higher than that of a compressor using chlorofluorocarbon as the refrigerant. Thus, the fluctuating range of the suction pressure is much wider in a compressor using CO.sub.2. Accordingly, the solenoid 6 must be excited within a wide current altering range to correspond to the wide fluctuating range of the suction pressure. To tolerate such wide current altering range, a large control valve 1 must be employed. However, this increases the size and weight of the compressor.
Japanese Unexamined Patent Publication No. 6-341378 describes a variable displacement compressor that employs an electromagnetic control valve that adjusts the difference between the pressure of the control chamber and the pressure of the suction chamber to an arbitrary constant value. This control valve also has two pressure chambers, which are partitioned from each other by a pressure sensing element. The pressure of the control chamber is communicated to one chamber, while the pressure of the suction chamber is communicated to the other chamber. The pressure sensing element moves the valve body in accordance with the fluctuations of the pressure in the control chamber and the pressure in the suction chamber. This maintains a constant difference between the pressure of the control chamber and the pressure of the suction chamber. Furthermore, the target pressure difference is varied in accordance with the value of the current fed to an electromagnetic solenoid, which is arranged in the control valve.
This control valve need only alter the value of the current fed to the solenoid within a range that corresponds to the fluctuating range of the pressure difference. The fluctuation range of the pressure difference is much more narrow than the fluctuation range of the suction pressure. This permits the range of the current fed to the solenoid to be much more narrow that of the control valve illustrated in FIG. 6. Thus, a smaller control valve can be employed.
However, in this control valve, an increase in the pressure of the suction chamber, or an increase in the thermal load (cooling load), opens the valve more. This increases the amount of refrigerant gas that flows into the control chamber from the discharge chamber. Therefore, the pressure in the control chamber increases in accordance with the increase of the pressure in the suction chamber. So the increase in the displacement of the compressor does not occur when the pressure in the suction chamber increases. An increase in the thermal load leads to an increase in displacement. However, the control valve keeps approximately the same displacement as the thermal load increases. This prevents the control valve from being employed to control displacement by directly using suction pressure, which reflects the thermal load. To control displacement in accordance with the suction pressure, a sensor can be employed to detect the suction pressure and output a corresponding electric signal. In this case, the signal is sent to a controller that controls the electric current fed to the solenoid based on the suction pressure. However, such a structure is complicated.